This invention relates to the manufacture of multilayer printed circuit boards and, more specifically, to a method for forming interconnections between layers of such printed circuit boards.
Prior art multilayer printed circuit (P.C.) boards have used via holes drilled in order to interconnect circuitry between the plural layers. On state-of-the-art boards, the vias are typically 1 mm (0.04") diameter or less. These vias are individually plated through and connections are effected by causing (horizontal) circuit traces on the different layers to reach the vias. If such boards were to be modified by selectively connecting different layers in accordance with the specific modification, then circuit traces on ers intermediate the connection had to be removed either in fabrication or by broaching. Due to the small diameter of the via holes, such broaching was difficult and was liable to cause circuit continuity defects.
It is frequently desired to effect a series of connections between layers wherein access to the connections is readily available. This requires that a large vertical hole, having a cross-sectional area greater than that of a typical via, be created and vertical connections formed to extend along the perimeter of the hole. In essence, such a hole would be an oversized via hole having plural vertical circuit traces extending along the perimeter of the hole. These circuit traces could be selectively broached with relative ease because the size of each trace is relatively small compared to the size of the hole. Furthermore, connections with or between the vertical traces can be readily made by the insertion of a rod or similar device, having circuit traces thereon, into the hole.
It is desirable to form vertical circuit traces within a small horizontal area such that the vertical circuit traces are relatively easily accessible by mechanical means. By providing for mechanical accessibility, connections can be broken to selectively leave traces connected at plural layers of a multilayer circuit board after plating operations have been completed by merely broaching a plated connection from the external surface of a vertical opening. It is further desired to be able to form a vertical opening to be significantly larger than the size of vertical traces extending along the perimeter of the hole.